1. Field of the Invention
The present invention generally relates to quadrupole array based residual gas sensor and, in particular, is concerned with a residual gas sensor which utilizes a miniaturized quadrupole array to sense the presence of certain gases within low pressure chambers and a process for manufacturing the same.
2. Description of the Prior Art
Quadrupole residual gas sensors are well known in the art and are used for detecting the presence of specific gases within a chamber in near vacuum conditions, e.g., at pressures of 1.times.10.sup.-5 Torr or below. The typical prior art quadrupole residual gas sensor includes four parallel rods, with equal lengths, precisely arranged and mounted on a ceramic base in a square configuration, thus forming a quadrupole, with an open area, or channel, at the center and extending the full length, of the rods. An electron source generates electrons at one end of the quadrupole which collide with, and ionize, some of the remaining gas molecules in the chamber. Some of these ions are then accelerated through the channel toward a collector positioned at the other end of the quadrupole. The ions that impact upon the collector generate a voltage potential upon the collector proportional to the number of ions and thus proportional to the population of gas molecules within the chamber. When the collector is connected to external circuitry, a current, proportional to the amount of ions impacting upon the collector is thereby generated.
Voltages are induced on the four parallel rods comprising the quadrupole. These voltages are tuned to generate an electric field in the channel between the four rods which permits only ions with a specific mass-to-charge ratio to travel the full length of the channel to the collector. Ions with other mass-to-charge ratios are pulled by the electric field from the channel to one of the four parallel rods and neutralized. Hence, by tuning the voltages on the rods for different mass-to-charge ratios, and by analyzing the current generated by ions impacting on the collector at these voltages, the quadrupole can be used to detect the presence of different gases within a chamber under low pressure or near vacuum conditions. The ability to sense these gases is important for such applications as thin-film deposition in semiconductor device processing as the presence of a specific gas in a near vacuum chamber during thin-film deposition may result in ruined devices.
For a quadrupole residual gas sensor to be able to operate in the above manner, the rods comprising the quadrupole must be precisely mounted with each of the rods parallel to each other and exactly located in the square quadrupole configuration. Heretofore, these rods have been mounted in holes precision drilled in a ceramic base. To achieve sufficiently precise positioning of the rods, as well as to maintain the low pressure integrity of the sensor, the holes typically have to be machine drilled to extremely low tolerances, e.g., 0.2 mil. The rods must then be precisely positioned within these holes in the ceramic base in the parallel, quadrupole configuration. The rods are typically secured to the ceramic base by either nuts or screws which must be precisely tightened to exact torque measurements to avoid any shifting of the rods from the parallel quadrupole configuration. Further, the electrical connections to the rods, as well as the mounting of other components on the sensor must also be made in an extremely precise and delicate fashion to ensure that the rods remain in the exact quadrupole configuration.
Unfortunately, the precision drilling of the ceramic base and the precision mounting of the rods during assembly make prior art quadrupole residual gas sensors extremely expensive to manufacture. Consequently, prior art quadrupole residual gas sensors are typically very expensive to buy, so expensive in fact, that when the sensors become dirty after continued operation, they are usually disassembled and cleaned rather than replaced with a clean sensor. However, after cleaning, re-assembling the sensor still involves precise and careful mounting and handling of the components of the quadrupole. Hence, while cleaning the sensor is less expensive the replacing the sensor, cleaning the sensor is still very expensive.
Further, the extremely precise tolerances needed to construct the sensor with the ceramic base requires larger sensor components. Specifically, since screws and/or nuts are used to secure and seat the rods within the holes drilled in the ceramic base, the rods must have a sufficient diameter to permit the attachment and tightening of these nuts and screws. For these reasons, the cylindrical rods used to construct prior art quadrupole assemblies typically are at least a 1/4" in diameter.
One consequence of using large diameter rods mounted in a ceramic base to construct a quadrupole residual gas sensor is that the rods must be spaced farther apart in order to obtain a channel between the rods where the electric field can be tuned for ions having a specific mass-to-charge ratio. However, if the rods are farther apart, the electric field produced by each rod must still be the same in order to cause ions with the wrong mass-to-charge ratio to leave the channel. Unfortunately, however, expensive equipment is required to produce such high voltages.
Due to the difficulties and costs associated with manufacturing the above-described prior art quadrupole sensor, existing sensors are generally limited to a single four-rod quadrupole. An array of quadrupoles can be used to obtain a highly sensitive residual gas sensor. While an array of quadrupoles has been previously been suggested in a paper entitled Das elektrische Massenfilter als Massenspektrometer und Isotopentrenner, Paul, et al., Zeitschrift fur Physik, Bd., Apr. 21, 1958, the practical difficulties and high cost described above with constructing a sensor with just one quadrupole effectively prevents construction of a cost effective sensor incorporating an array of quadrupoles. Specifically, the cost of precisely drilling holes in a ceramic base to accommodate an array of rods, and the cost of precisely positioning the rods, effectively prevent the manufacture of an affordable quadrupole array based sensor. Further, as described above, the rods comprising the array would still have to be large diameter rods, spaced relatively far apart. Consequently, the size of an array of quadrupoles manufactured using the known techniques would be sufficiently large to limit its use in most low pressure or vacuum chambers.
Currently, the selectivity of the single prior art quadrupole sensor described above can only be improved by both increasing the length of the rods to lengthen the distance the ions must travel to the collector, and by increasing the frequency of the AC component of the voltages applied to the rods to create a more rapidly fluctuating electric field.
Typically, prior art quadrupole residual gas sensors have rods about 4 to 6 inches long. To maximize the sensitivity of the sensor, however, the length the ions must travel in the channel to the collector must be less than the mean free path of the ions. The mean free path of an ion is the mean distance the ion will travel in a straight line through its environment prior to colliding with another molecular particle. The channel length must, preferably, be less than the mean free path of the particle to thereby minimize the likelihood of an ion, with the tuned mass-to-charge ratio, colliding with another particle and being deflected out of the channel or neutralized. Tuned ions which are deflected in this manner will not impact upon the collector, resulting in a lower current being detected at the collector. The mean free path of a particle, such as an ion, can be calculated by a well known formula in which the mean free path is inversely proportional to the pressure of the environment that the particle is in. Hence, prior art quadrupole residual gas sensors must operate at extremely low pressures, e.g., 5.times.10.sup.-5 Torr, to be able to obtain a mean free path greater than the length of the channel between the ion source and the collector.
In many applications where there is a need to determine what gases exist in a chamber, the pressure in the chamber is substantially higher than the pressures necessary to operate the prior art sensor. For example, in the film deposition techniques used in the manufacture of semiconductor devices, the films are often deposited in chambers where the pressure may even be two orders of magnitude greater than the pressure needed to operate the above-described prior art sensors.
Consequently, the user is then reduced to sampling the contents of the low pressure chamber into a separate chamber, and then lowering the pressure in the separate chamber to obtain the pressure needed for the sensor to operate. As can be appreciated, the additional hardware necessary to implement such sampling is very expensive, and sampling is inherently inaccurate. Further, in these applications, the quadrupole residual gas sensor is not embedded in the low pressure chamber where the gas is being sensed, it is mounted in an extraneous chamber.
Consequently, there is a need in the prior art for an inexpensive residual gas sensor which uses an array of quadrupoles to increase sensitivity. Further, there is an additional need in the prior art for a sensor capable of operating at higher pressures to eliminate the costs and inaccuracies associated with sampling the contents of a low pressure chamber and to permit the sensor to be directly embedded in the chamber. Finally, there is a need in the prior art for both an inexpensive method of manufacturing these improved sensors, and an apparatus to facilitate such manufacturing.